1. Field of the Invention
The present invention relates generally to devices and methods used for mixing and cleaning in microfluidic systems. More particularly, this invention pertains to employing electrothermally induced flow to enhance mixing of chemical and biological samples and cleaning in microscale devices.
2. Description of the Prior Art
Miniaturized bioanlytical or Bio-Micro Electro Mechanical Systems (“BioMEMS”) (hereafter collectively referred as microdevices) are used to perform various functions such as a simple mixing of two or more analytes or liquid streams (hereafter collectively referred as samples) to a more complex biochemical assay that can include immunoassays, DNA hybridization, and general cell-molecule interactions. These devices incorporate many of the necessary components on a single platform, known as a biochip or microfluidic chip (hereafter collectively referred as microfluidic system). The term “microfluidic” is commonly used if at least one characteristic dimension of the device is in micron size. Typical biochip components known in the art include reaction chambers, pumps, micromixers, pre-concentrators, interconnects, separators, and sensors. The successful implementation of a biochemical assay using a microfluidic system is determined in terms of parameters that can include overall assay time, recovery time, sensitivity, selectivity, and accuracy.
In microdevices, samples are usually mixed as a part of an assay protocol. The time taken to accomplish this task, known as “mixing time”, is determined by the diffusion coefficient (usually a very small value) of the samples, their flow speed, and residence time inside the device. This time can form a significant portion of the “overall assay time”. In this regard, there is a need for methods and systems that will facilitate rapid mixing so that overall assay time may be reduced. Preferably, such devices should contain no moving parts.
A second performance parameter is the recovery time, which is defined as the time taken for the device to get ready before analyzing next set of samples. This requires cleaning of the device. In this aspect also, there is a similar need for systems and methods that will facilitate efficient cleaning.
Most conventional micromixing systems can be classified as either active or passive. Passive mixers use molecular diffusion of samples, and consequently take a very long time to accomplish mixing. Active mixers use externally imposed forcing mechanisms, such as a pressure pulse or an oscillatory flow, and therefore take a relatively short time to accomplish mixing. Known methods of micromixing include electroosmotic flow (electrohydrodynamic instabilities), static lamination (diffusional forces as mixing mechanism), and injection of one liquid into another liquid with microplumes.
Passive mixers do not have any moving parts, in contrast to active devices where moving parts are activated either by a pressure or by an electric field. Passive mixers use channel geometry to increase residence time. Passive micromixers are further subdivided into in-plane and out-of-plane mixers. In-plane mixers divide and mix various liquid streams in one dimension while out-of-plane mixers use three-dimensional channel geometries to enhance mixing. The simplest passive in-plane mixer is a one that merges two different liquid streams into a single channel and accomplishes mixing via molecular diffusion.
Cleaning methods that are conventionally practiced in the industry include ultrasonic cleaning and vacuum washing.
What is needed, then, are methods and systems for inducing flow in a microfluidic system that use no moving parts, are easy to control, and that do not require special treatment of surfaces in the system.